1. Field
The following description relates to a semiconductor device, and to, for example, a semiconductor device having a trench whose upper width is wider than a lower width thereof, and a method for fabricating the same.
2. Description of Related Art
Metal oxide semiconductor field effect transistor (MOSFET) is a type of semiconductor device that is well known in the semiconductor industry. One type of MOSFET is a vertically-conducting trench MOSFET.
FIG. 1 is a sectional view schematically illustrating a MOSFET.
Referring to FIG. 1, a MOSFET 10 includes trenches 17. The trenches 17 each include a polysilicon gate 21 that is insulated from a body region 15 by a gate insulating layer 19. A source region 23 contacts a side surface of each trench 17. The gate insulating layer 19 insulates the gate 21 from a metal layer 30. A semiconductor substrate 11 forms a drain of the MOSFET 10.
Still referring to FIG. 1, a reference numeral A denotes a trench width, B denotes a contact opening, and C denotes a gap between the trench and the contact opening. When the MOSFET 10 is biased in an ON state, a current vertically flows between the source region 23 and the semiconductor substrate 11.
In the ON state, a current capability of the MOSFET 10 is in reverse proportion to ON resistance Rdson between drain and source. Hence, to improve the current capability of the MOSFET 10, the ON resistance Rdson has to be lowered.
One method of lowering the ON resistance of the MOSFET 10 is to increase density of the trench 17, namely, the number of trenches per unit area. This is achieved by reducing a cell pitch. However, the degree to which the cell pitch of the MOSFET may be reduced is limited by certain features included in the MOSFET cell and the processes used for fabricating the MOSFET.
In a MOSFET, a source region is generally formed on a semiconductor substrate in a horizontal direction from a trench to lower the ON resistance. This increases the density of the MOSFET cell per unit area.
In order to lower the ON resistance of the MOSFET, the number of trenches per unit area has to be increased. However, a process of setting a gap between the trench and the contact opening on a surface is required in order to form the source region on the semiconductor substrate in the horizontal direction from the trench. Consequently, there is a limit to the degree to which the trench gap may be reduced, and this further limits the degree to which the ON resistance of the MOSFET may be lowered.